Page 45 - Chemistry ICSE Class IX
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Chemical Changes and Reactions 33
AgNO (s) + NaCl(s) o No reaction
3
AgNO (aq) + NaCl(aq) o AgCl(s) + NaNO (aq)
3
3
silver nitrate sodium chloride silver chloride (white ppt) sodium nitrate
Particle size
The rate of reaction also depends upon particle size of the reactants. Smaller
the particle size of the reactants, faster would be the reaction. Lumps of coal
CTG FKHſEWNV VQ DWTP KP CKT DWV EQCN RQYFGT DWTPU KP CKT CNOQUV GZRNQUKXGN[. The dry grass does not catch
This is because ne powders have very large surface area as compared to fire when matchstick is kept at a
the lump. As a result, larger surface area is available for the contact between distance
the reactants and hence, the reaction is fast.
ACTIVITY 1 WůĂĐĞ Ă ĨĞǁ ƉŝĞĐĞƐ ŽĨ ƉĂƉĞƌ Žƌ ĚƌLJ ŐƌĂƐƐ ŽŶ Ă ƐƚŽŶĞ ƐůĂď͘ ,ŽůĚ Ă ďƵƌŶŝŶŐ ŵĂƚĐŚƐƟĐŬ Ăƚ ƐŽŵĞ ĚŝƐƚĂŶĐĞ ĂǁĂLJ ĨƌŽŵ ƚŚĞŵ͘
dŚĞ ƉŝĞĐĞƐ ŽĨ ƉĂƉĞƌ Žƌ ŐƌĂƐƐ ĚŽ ŶŽƚ ĐĂƚĐŚ ĮƌĞ͘ EŽǁ ďƌŝŶŐ ƚŚĞ ŵĂƚĐŚƐƟĐŬ ŶĞĂƌĞƌ ƚŽ ƚŽƵĐŚ ƚŚĞ ƉŝĞĐĞƐ ŽĨ ƉĂƉĞƌ ;Žƌ ŐƌĂƐƐͿ͘
tŚĂƚ ĚŽ LJŽƵ ƐĞĞ͍ WĂƉĞƌ ƉŝĞĐĞƐ ;Žƌ ŐƌĂƐƐͿ ĐĂƚĐŚ ĮƌĞ ĂŶĚ ďƵƌŶ ƚŽ ůĞĂǀĞ ĂƐŚ ďĞŚŝŶĚ͘ dŚŝƐ ƐŚŽǁƐ ƚŚĂƚ ĨŽƌ Ă ĐŚĞŵŝĐĂů ĐŚĂŶŐĞ
ƚŽ ƚĂŬĞ ƉůĂĐĞ
ŚĞĂƚ ;ŝŶ ĐĞƌƚĂŝŶ ĐĂƐĞƐͿ ŝƐ ƌĞƋƵŝƌĞĚ ƚŽ ƐƚĂƌƚ ƚŚĞ ĐŚĂŶŐĞ͘
ƚŚĞ ƌĞĂĐƟŶŐ ƐƵďƐƚĂŶĐĞƐ ƐŚŽƵůĚ ĐŽŵĞ ŝŶ ĐŽŶƚĂĐƚ͘
Heat
Many reactions either do not occur or are too slow at lower temperatures. On
heating (or at higher temperature), the reaction proceeds at an appreciable
rate.
For example,
Limestone (CaCO ) decomposes when heated above 1273 K (or
3
1000°C) to give quicklime (CaO) and carbon dioxide (CO ). The dry grass catches fire when
2
1273 K
CaCO (s) o CaO(s) + CO (g) matchstick is brought closer to it
2
3
limestone lime (quicklime) carbon dioxide
Marble chips (CaCO ) and hydrochloric acid [HCl(aq)] react to give
3
carbon dioxide (CO ) and calcium chloride (CaCl ). The reaction
2
2
becomes much faster when warmed.
CaCO (s) + 2HCl(aq) o CaCl (aq) + H O(l) + CO (g)
3
2
2
2
marble chips hydrochloric acid calcium chloride water carbon dioxide
Aluminium displaces iron from iron(III) oxide on heating.
heat
Fe O (s) + 2Al(s) o Al O (s) + 2Fe(l)
3
2
2
3
iron(III) oxide aluminium aluminium oxide iron
Light
WŚŽƚŽŐƌĂƉŚŝĐ ƉůĂƚĞ ŐĞƚƐ ĂīĞĐƚĞĚ
Certain reactions take place only in the presence of light (or visible radiation). ǁŚĞŶ ĞdžƉŽƐĞĚ ƚŽ ůŝŐŚƚ͘ dŚŝƐ ŝƐ ĂůƐŽ Ă
Such reactions are called photochemical reactions. Light is absorbed by the photochemical reacƟon͘
reacting molecules. The reacting molecules get activated and react. The rate
of a photochemical reaction increases with the intensity of light.
For example, when a mixture of hydrogen and chlorine gases is exposed
to diffused sunlight, the two gases react at normal rate to form hydrogen
chloride. But when similar mixture is exposed to bright sunlight, the two
gases react explosively. This is because the rate of reaction increases with
the brightness (or intensity) of the sunlight.
diffused sunlight
H (g) + Cl (g) o 2HCl(g) — Normal reaction
2
2
bright sunlight
H (g) + Cl (g) o 2HCl(g) — Explosive reaction
2
2
